Description of the Prior Art
Generally, a positive or negative voltage, which is generated by a charge pump, is required to program or erase a flash memory device. The charge pump is classified into a positive charge pump and a negative charge pump. A conventional negative charge pump generates a negative voltage using a zero (0) through five (5) voltage. Such a negative charge pump has a drawback in that difference of voltage down is small. To overcome the drawback, a negative charge pump having a plurality of diodes and capacitors has been developed. The diodes are connected in series from each other and each capacitor is connected to a connection point of the diodes. The conventional negative charge pump has a drawback in that size of the memory device is increased due to a plurality of capacitors. Also, the conventional negative charge pump has another drawback in that it is difficult to obtain a desired negative voltage since each diode has a threshold voltage.